There are known various methods of separating a particle group whose particles (such as cells and chemically synthesized particles) are different in properties, for example, a method of separating a particle group on the basis of a difference in mass, a method based on a size of particles, and a method based on a difference in density.
Methods, such as a sedimentation equilibrium method and a sedimentation velocity method with use of a separation fluid whose density is regulated and a centrifugal separation method, are exemplified as a method of separating, in accordance with a difference in density, a particle group whose particles are different in density (see Patent Literature 1 through 6).
In a case where cells are separated as particles, there is a known method in which (i) magnetic particles or fluorescent molecules are attached to cells by using a specific antigen-antibody reaction which is an inherent immunological property of a biospecimen such as cells and (ii) the cells are separated by a cell separation device such as a magnetic cell sorter or a fluorescence-activated cell sorter.
Further, there is studied another method in which electrophoresis or a flocculant is used in a micro flow path (see Patent Literature 7). Furthermore, there have been developed a device for separating a particle group in a micro flow path on the basis of a density with use of an ultrasonic wave or a pinched flow path (see Non-Patent Literatures 1, 2) and another method of classifying a minute particle group (see Patent Literatures 8, 9).